Plaque tear extending from intima into media with dissection was observed only in the early and late success groups (P=0.03). Hemorrhage into plaque was present in 16 (80%) of 20 PTCA sites in the 2 early groups and in 3 (37%) of 8 sites in the late group (P<0.03). Occlusive thrombus (5 of 16, 1 of 4, and 1 of 8) and plaque debris (7 of 16, 1 of 4, and 2 of 8) in residual lumens were insignificantly different among the 3 groups and their 82 five-mm segments. Plaques that had >25% lipid content, however, had an increased frequency of hemorrhage into plaque (P<0.004), occlusive thrombus (P=0.0001) and plaque debris in residual lumens (P<0.05). The findings in this study suggest that coronary arterial size and plaque composition are strong determinants of PTCA outcome. The ideal coronary arterial atherosclerotic narrowing for both mechanically and clinically successful PTCA appears to be a small (<3.3 mm in internal diameter) artery in which the plaque contains relatively little calcium and lipid.